The present invention relates to a process for the preparation of polymers via controlled xe2x80x9cpseudo-livingxe2x80x9d free-radical polymerization of vinyl monomers using nitrogen monoxide.
A process for the production of polymers by means of pseudo-living free-radical polymerization is disclosed. The process entails (i) reacting a polymerizable vinyl monomer with nitrogen monoxide and at least one initiator to prepare a free-radical intermediate product, and (ii) polymerizing the intermediate product optionally together with one or more additional monomers and/or with a free-radical initiator.
FIG. 1 shows the relationship between number average molecular weight and conversion for polymerizations with and without and activator according to the invention;
FIG. 2 shows the relationship between polydispersity (Mw/Mn) and conversion for polymerizations with and without and activator according to the invention;
FIG. 3 shows the relationship between number average molecular weight and conversion for polymerizations according to the invention;
FIG. 4 shows the relationship between number average molecular weight and conversion for polymerizations according to the invention;
FIG. 5 shows the relationship between number average molecular weight and conversion for polymerizations according to the invention;
FIG. 6 shows the relationship between number average molecular weight and polydispersity versus conversion for polymerizations according to the invention;
FIG. 7 shows the relationship between number average molecular weight and conversion for polymerizations according to the invention;
FIG. 8 shows the relationship between number average molecular weight and conversion for polymerizations according to the invention;
FIG. 9 shows the relationship between number average molecular weight and conversion for polymerizations according to the invention;
FIG. 10 shows the relationship between number average molecular weight and time for polymerizations according to the invention;
FIG. 11 shows the relationship between number average molecular weight and conversion for polymerizations according to the invention; and
FIG. 12 shows the relationship between number average molecular weight and time for polymerizations according to the invention.
Nowadays, the demand for homopolymers, random copolymers and block copolymers of a specific molecular weight, a narrow molecular weight distribution and/or well-defined end groups has continuously increased in a number of industries. The controlled structure of these macromolecules provides them with novel properties and allows a tailor-made property profile to be obtained. Thus many new technologies require controlled polymer structures, such as for example in the fields of electronics, computer science, communications, genetic engineering, biotechnology and materials science.
Well-defined polymers can for example be produced by ionic living polymerization processes. However, ionic processes require drastic reaction conditions, such as for example temperatures of about xe2x88x9278xc2x0 C., extreme dryness and the absence of protic species and only a few monomers can be used.
In contrast to ionic processes, free-radical polymerization can be carried out under mild conditions and a wide range of monomers can be used. Significant progress in the control of free-radical polymerization has been made.
Otsu et al., for example, were the first to report on the possible control of the free-radical polymerization of some vinyl monomers based on the iniferter concept (see: J. Polym. Sci.: Part A: Polym. Chem. 2000, 38, 2121). Meanwhile three main methods of controlled free-radical polymerization, sometimes described as xe2x80x9clivingxe2x80x9d free-radical polymerization, have been developed:
(I) Nitroxide-mediated polymerization (NMP), which is based on the reversible capture of the propagating radicals by nitroxide radicals to form dormant chains. This strategy is disclosed for example in U.S Pat. No. 4,581,429. This process is handicapped by slow polymerization kinetics, a limited range of suitable monomers and the high cost of the required nitroxide radicals.
(II) Atom transfer radical polymerization (ATRP), which involves the reversible trapping of the propagating chains by halogen atoms and is mediated by metallic catalysts, is described extensively in Acc. Chem. Res. 1999, 32, 895. A serious drawback of this method is the use of transition metal catalysts (Fe, Cu) which are potentially toxic, impart colour to the resulting polymers and lead to corrosion problems. Removal of the catalyst from the resulting polymers is possible but relatively costly.
(III) Reversible addition-fragmentation chain transfer (RAFT) using dithio esters as transfer agents, which is disclosed for example in WO 98/01478. This method is limited to specific monomers and uses costly dithio esters which impart an unpleasant smell to the resulting polymers.
Although controlled free-radical polymerization represents an attractive method of obtaining new polymer structures there is still a need for an inexpensive, versatile control method that can be applied to a broad range of monomers.
One method of obtaining alkoxyamine initiators as nitroxide precursors has been described in WO 98/13392 and in EP-A 0 891 986. Conventional free-radical initiators are decomposed thermally in the presence of nitrogen monoxide NO. The resulting NO compounds can be used as initiators for controlling the free-radical polymerization of vinyl monomers. A disadvantage of these initiators is that polymerization appears to be controlled only for short polymer chains with number average molecular weights smaller than 15,000. Most industrially relevant polymers have to possess a number average molecular weight in excess of 50,000 in order to display the desired mechanical properties such as rigidity, elasticity or toughness. In addition, the methods described in the cited prior art still require the synthesis and isolation of the initiators in a step separate from the polymerization.
When added directly during free-radical polymerization nitrogen monoxide is reported (such as for example in DE-A 2 660 230, U.S. Pat. Nos. 4,040,912 and 3,964,979) to be a polymerization inhibitor.
Under appropriate conditions sodium nitrite may be used as a precursor of nitroxide compounds. Since sodium nitrite is inexpensive, this could provide a cheap method of access to nitroxide radicals.
The prior art has already mentioned sodium nitrite as a control agent for free-radical polymerization. For instance, EP-A 1 061 059 mentions sodium nitrite as an inhibitor of the free radical polymerization of butadiene when used in combination with specific phosphorus-containing compounds. The possible control of polymerization under such conditions is not disclosed.
In attempts to reduce gel formation during the polymerization of concentrated solutions of sodium acrylate in water, Bortel et al. (J.M.S.-Pure Appl. Chem. 1998, A35(2), 401) observed that adding sodium nitrite inhibits polymerization up to a polymerization temperature of 60xc2x0 C. It was found that although the molecular weight of the polymer increases slightly with time, the polydispersity (D={overscore (M)}w/{overscore (M)}n) is very high (typically D higher than 6), this being a clear indication of xe2x80x9cclassicalxe2x80x9d free-radical polymerization. Other monomers than sodium acrylate are not disclosed.
The in-situ formation of NO compounds from free radical initiators and nitroso compounds is reported in Mendeleev Comm. 1999, 6, 250. Similar compounds can also be formed by the reaction of free radical initiators with nitrones as reported in WO 99/03894. These methods still however require the use of preformed reagents which may be toxic.
The present invention is based on the problem of providing homo- and copolymers of controlled molecular weight and controlled molecular structure by a simple and inexpensive method of controlling the free-radical polymerization of vinyl monomers that overcomes the drawbacks encountered in the prior art.
It has now been found that the free radical polymerization of vinyl monomers and of vinyl monomer mixtures can be easily controlled with respect to the molecular weight or molecular weight distribution of the resulting homopolymers, random and block copolymers if it is conducted in the presence of a reaction product of at least one vinyl monomer with nitrogen monoxide and at least one free-radical initiator.
The present invention relates to a process for producing polymers by means of controlled pseudo-living free-radical polymerization of olefinically unsaturated monomers using nitrogen monoxide.
In the present process, in a first step a free-radical intermediate product of the general formula (I), 
in which
M represents a vinyl monomer,
p is an integer of 1 to 15 ,
q is an integer of 1 to 15 and
Init1 and Init2 independently one of the other denote a radical of a free-radical initiator,
is produced by reacting at least one vinyl monomer with nitrogen monoxide and at least one initiator and then in a second step the polymerization is carried out optionally with the addition of an additional monomer or monomer mixture and/or free-radical initiator.
The free-radical intermediate product of the general formula (I) may optionally be isolated. Preferably the process according to the invention is carried out in the form of a one-pot synthesis.
The polymerization process of the present invention may be used to prepare various polymers of low polydispersity, which include for example end-functionalized polymers, block, multiblock or gradient polymers, star-shaped polymers or graft and branched polymers.
The first and second steps of the polymerization may generally be carried out in different temperature ranges. The polymerization step of the process according to the invention may also optionally be carried out in several stages, it being possible to add the required monomer or monomer mixture at any stage. Using this method block copolymers may for example be produced. The excess monomer or monomer mixture may optionally be removed at the end of each stage before continuing with the addition of further monomer.
Monomers or combinations of monomers known for free-radical polymerization may be polymerized according to the invention.
Monomers M that may be used in the first step of the present invention are any polymerizable vinyl monomers such as styrene, xcex1-methylstyrene, p-methylstyrene and other alkylstyrenes, acrylic and methacrylic acid and alkyl esters thereof, (meth)acrylamide, (meth)acrylonitrile, acrolein, vinyl acetate, vinyl chloride, conjugated dienes and vinylpyridine. Preferred monomers are styrene, xcex1-methylstyrene and alkylstyrenes, acrylic and methacrylic acid and alkyl esters thereof, acrylonitrile, butadiene or isoprene. Particularly preferred monomers are acrylic and methacrylic alkyl esters and acrylonitrile. In particular M may also represent different vinyl monomers, such as for example where the compound (I) is formed by reacting nitrogen monoxide with at least one initiator and a mixture of vinyl monomers.
Typical monomers M suitable for the second step of the present invention are for example styrene, xcex1-methylstyrene, p-methylstyrene and other alkylstyrenes, acrylic and methacrylic acid and alkyl esters thereof, (meth)acrylamide, (meth)acrylonitrile, acrolein, vinyl acetate, vinyl chloride, conjugated dienes, vinylpyridines, maleic acid and the anhydride thereof and fumaric acid. Preferred monomers are styrene, xcex1-methylstyrene and alkylstyrenes, acrylic and methacrylic acid and alkyl esters thereof, acrylonitrile, butadiene, isoprene and maleic anhydride. Particularly preferred monomers are acrylic and methacrylic acid and alkyl esters thereof, styrene, xcex1-methylstyrene, acrylonitrile and butadiene.
In the process according to the invention nitrogen monoxide is for example either introduced into the reaction medium directly in the form of a nitrogen oxide gas or produced by an in situ reaction between a nitrite compound and an activator component. Suitable activator components are for example reducing or oxidizing agents or acids.
The gaseous nitrogen monoxide used in the process according to the invention may be used in pure form or as a mixture with gaseous nitrogen oxides of the general formula (II)
Nx Oy xe2x80x83xe2x80x83(II) 
in which
a) x is 1 and y is 2 or
b) x is 2 and y is 1,3,4 or 5,
in any desired ratios between the monoxide and (II).
In a preferred embodiment of the invention nitrogen monoxide is produced in situ by a reaction between a nitrite compound and an activator component.
Suitable nitrite components are both inorganic and organic nitrite compounds. Inorganic nitrite compounds are for example compounds of the general formula (III),
Z(NO2)n xe2x80x83xe2x80x83(III) 
in which
Z is selected from the group consisting of alkaline, alkaline earth, earth or transition metals or lanthanide or actinide or from one of the cations Sn2+, Sn4+, In3+, Tl+, Tl3+, Pb2+, Bi3+, Sb3+, Sb5+ or NH4+ and
n is 1 to 5.
Suitable organic nitrite compounds are for example compounds of the general formula (IV),
Rxe2x80x94Oxe2x80x94Nxe2x95x90O xe2x80x83xe2x80x83(IV) 
in which
R is an organic radical containing at least 2 carbon atoms and other functional groups, selected from the group comprising esters, thioesters, ethers, thioethers, alcohols, thiols, amines, amides, imines, imides, urethanes, halogen, nitro, nitroso, nitrates, nitrites, sulfates, sulphones, nitriles, carboxylic acids, carboxylic anhydrides, aldehydes and ketones.
Preferred compounds of formula (IV) are for example water-soluble salts, where Z is a mono- or divalent cation. Particularly preferred are for example those where Z is an alkaline cation or NH4+.
Preferred compounds of formula (IV) are for example those where R is an organic radical containing 2 to 6 carbon atoms. Particulary preferred compounds of formula IV are for example isoamyl nitrite, propyl nitrite, n-butyl nitrite or tert-butyl nitrite.
Suitable reducing agents for use as activators for the present invention are all known reducing agents, such as for example Fe(II), Cu(I) and Ti(III) salts, ascorbic acid, formic acid, tartaric acid, oxalic acid and salts thereof, alkaline hydroxymethanesulfinate salts, dextrose, formaldehyde, hydrogen peroxide, sodium sulfite, sodium and potassium thiosulfate, 2-mercaptoethanol and other thiols. In a preferred embodiment the reducing agents are Fe(II) salts, ascorbic acid, formic acid, oxalic acid, formaldehyde or dextrose. Particularly preferred reducing agents are Fe(II) sulfate, ascorbic acid and dextrose.
Suitable oxidants for use as activators for the present invention are for example Fe(III), Cr(III), Cu(II), Ti(IV), Sn(IV), Hg(II) salts, alkaline permanganates, alkaline chromates, hydrogen peroxide, alkaline hypochlorites. Preferred oxidants are Fe(III), Cu(II) and Cr(III) salts. Particularly preferred oxidants are Fe(III) and Cr(III) sulfates.
Suitable acids for use as activators are for example strong and weak inorganic and organic acids, such as for example hydrochloric acid, sulfuric acid, phosphoric acid, formic acid, ascorbic acid, acetic acid, tartaric acid, oxalic acid and methanesulfonic acid. In a preferred embodiment the acid is for example sulfuric acid, phosphoric acid, acetic acid or methanesulfonic acid. Particularly preferred acids are sulfuric and acetic acids.
The activator or mixture of activators is used in amounts of 0.01 to 500 mol %, preferably 0.1 to 300 mol %, most preferably 1 to 200 mol %, based on the nitrite component.
Suitable free-radical initiators for both steps of the present invention are any suitable agents producing free radicals, including precursors such as azo compounds, peroxides or peroxy esters, which generate radicals for example by thermolysis, or precursors such as styrene, which generate radicals by autopolymerization. Also suitable are redox initiating systems such as K2S2O8+FeSO4, photochemical initiating systems or high energy radiation such as electron beam or X- or xcex3-radiation. In the latter case the molar amount of radicals generated in the polymerization medium represents the molar amount of initiator and may be calculated based on the total amount of radiation energy and the radical generation efficiency of the type of radiation employed.
Initiators may also be molecules containing free radicals such as oxygen, nitrogen dioxide and ozone.
Examples of free radical initiators generating radicals by thermolysis are 2,2xe2x80x2-azobis(isobutyronitrile), 2,2xe2x80x2-azobis(isovaleronitrile), 2,2xe2x80x2-azobis(methyl isobutyrate), 4,4xe2x80x2-azobis(4-cyanopentanoic acid), 1,1xe2x80x2-azobis(1-cyclohexanecarbonitrile), 2-tert-butylazo-2-cyanopropane, 2,2xe2x80x2-azobis[2-methyl-N-(1,1-bis(hydroxymethyl)-2-hydroxyethyl propionamide], 2,2xe2x80x2-azobis[2-methyl-N-(2-hydroxyethyl) propionamide], 2,2xe2x80x2-azobis (isobutyramidine hydrochloride), 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethylene isobutyramine), 2,2xe2x80x2-azobis[2-methyl-N-(1,1-bis(hydroxymethyl)-2-ethyl)-propion-amide], 2,2xe2x80x2-azobis[2-methyl-N-(2-hydroxyethyl)propionamide], 2,2xe2x80x2-azo-bis(isobutyramide)dihydrate, 2,2xe2x80x2-azobis(2,2,4-trimethylpentane), 2,2xe2x80x2-azobis(2-methylpropane), tert-butyl peroxyacetate, tert-butyl peroxybenzoate, tert-butyl peroxyoctoate, tert-butyl peroxyneodecanoate, tert-butyl peroxyisobutyrate, tert-amyl peroxypivalate, tert-butyl peroxypivalate, diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, dicumyl peroxide, dibenzoyl peroxide, dilauroyl peroxide, potassium peroxydisulfate, ammonium peroxydisulfate, di-tert-butyl hyponitrite and dicumyl hyponitrite.
Initiators generating radicals by photolysis include for example benzoin derivatives, benzophenone, acyl phosphine oxides and photoredox systems.
Initiators generating radicals as a result of a redox reaction consist in general of a combination of an oxidant and a reducing agent. Suitable oxidants are for example potassium peroxydisulfate, hydrogen peroxide, tert-butyl hydroperoxide, cumyl hydroperoxide, benzoyl peroxide and p-menthane hydroperoxide. Suitable reducing agents are for example Fe(II) salts, Ti(III) salts, potassium thiosulfate, potassium bisulfite, ascorbic acid and its salts, oxalic acid and salts thereof, hydrogen peroxide and dextrose.
The first step of the process according to the invention is carried out at a temperature between 0 and 120xc2x0 C., preferably between 20 and 100xc2x0 C., and most preferably between 40 and 80xc2x0 C.
In the second step the polymerization is carried out in the presence of the intermediate product formed in the first step at temperatures between 20 and 180xc2x0 C., preferably between 50 and 150xc2x0 C., and most preferably between 60 and 130xc2x0 C.
The monomer is added in an amount of at least 100 mol %, preferably at least 300 mol %, and most preferably at least 500 mol %, based on nitrogen monoxide.
Where the intermediate product is formed in a completely separate step from the polymerization step, such as for example when the intermediate product is isolated before further use, or where the monomer or monomer mixture to be polymerized is different from the monomer or monomer mixture used in the formation of the intermediate product, the monomer used in the formation of the intermediate product is added in an amount of at most 15,000 mol %, preferably at most 10,000 mol %, and most preferably at most 5,000 mol %, based on nitrogen monoxide.
Where the intermediate product is formed in a xe2x80x9cone-potxe2x80x9d process before the polymerization step is begun, the maximum amount of monomer or monomer mixture used in the first step may already be the amount of monomer required for the polymerization step and it depends on the molecular weight of the desired polymers. Typically this amount of monomer will be no more than 1,000,000 mol %, preferably no more than 500,000 mol %, and most preferably no more than 300,000 mol %, based on nitrogen monoxide.
If the monomer or monomer mixture to be polymerized is not present in the first step, the monomer or monomer mixture to be polymerized according to the invention is added in a later stage in amounts depending on the desired molecular weight of the polymer, and typically in amounts of between 500 and 1,000,000 mol %, preferably between 1,000 and 500,000 mol %, and most preferably between 2,000 and 300,000 mol %, based on the initiator used in the polymerization step.
The initiator compound is added in an amount of between 0.01 and 200 mol %, preferably between 0.05 and 150 mol %, and most preferably between 0.1 and 100 mol %, based on nitrogen monoxide.
The polymerization of the second step of the present invention is initiated with a free radical initiator or a mixture of free radical initiators, in amounts between 0.1 and 500 mol %, preferably between 0.5 and 200 mol %, and most preferably between 1 and 100 mol %, based on nitrogen monoxide used in the first step for the formation of the intermediate product.
The process may be carried out in the presence of an organic solvent or in the presence of water or in mixtures thereof. When a nitrite component is used as a source of nitrogen monoxide, water promotes the reaction between the nitrite component and the activator component. Additional cosolvents or surfactants, such as glycols or salts of fatty acids or salts of quaternary ammonium compounds or ethylene-oxide based amphiphilic compounds, may be used.
If organic solvents are used, suitable solvents or mixtures of solvents are typically pure alkanes, aromatic hydrocarbons, halogenated hydrocarbons, alkanols, esters, ethers and ketones or mixtures thereof. Preferred solvents are for instance toluene, tetrahydrofurane, ethyl benzene, diethylether, hexane and methyl ethyl ketone.
Water may be used in the process of the present invention and both water-soluble monomers and water-insoluble monomers may be used. The type of polymerization used may be bulk, solution, emulsion, dispersion or suspension polymerization and it may be carried out both batchwise and continuously.
The present invention also relates to intermediate products of the general formula (I) which are obtainable by the process according to the invention.
The intermediate products according to the invention of the general formula (I) may be present as mixtures with oligomeric nitroso compounds.
The invention also relates to polymers of the general formula (V), 
in which
Pol is a polymeric radical,
M represents a vinyl monomer,
p is an integer of 1 to 15 ,
q is an integer of 1 to 15 and
Init1 and Init2 independently one of the other denote a radical of a free-radical initiator.
The polymeric radical Pol contains structural units derived from the polymerization of the monomers used for step 2. Pol may be a linear-, block-, random-, graft- or star-copolymer.
The polymers according to the invention display a polydispersity D={overscore (M)}w/{overscore (M)}n less than 2, preferably D less than 1.8, and most preferably D less than 1.5.
The present invention also relates to the use of the intermediate product of the general formula (I) in free-radical polymerization.
The present invention also relates to the use of polymers of the general formula (V) for the production of coatings and as compatibilizers in thermoplastic polymer blends.
The polymerization process according to the present invention has several benefits over the prior art:
Polymers with low polydispersity D may be prepared compared to the high polydispersity observed in case of conventional free radical polymerization (D greater than 2). The polydispersity D of the polymers synthesized according to the invention is usually lower than 2 and may be significantly lower at low monomer conversion rates ( less than 30%).
Another aspect of the present invention is that the number average molecular weight of the polymer chains increases linearly with the monomer conversion, which allows a tailor-made polymer molecular weight to be obtained. Furthermore, the molecular weight of the polymers may be controlled by varying the amount of initiator and/or the amount of nitrogen monoxide and/or the amount of monomers used in the reaction.
A further benefit of the present invention is that after removal of the non-polymerized monomers or after reaching a conversion of 100%, a second polymerization step may be initiated simply by adding to the polymer synthesized in the first polymerization step a portion of fresh vinyl monomer or monomer mixture that may be different from the vinyl monomer or monomer mixture used in the first polymerization step. The polymerization of the vinyl monomer or monomer mixture added in the second step is then initiated by the polymer chains synthesized in the first polymerization step and di-block copolymers may for example be generated if the polymer chains synthesized in the first polymerization step were linear chains with one single growing site. The molecular weight and molecular weight distribution of each block may be controlled independently during the respective polymerization step. The process of polymerization of a vinyl monomer or monomer mixture initiated by polymer chains synthesized in a previous polymerization step carried out according to the invention may be repeated, whereby for example multi-block-copolymers of controlled molecular weight and molecular weight distribution for each block may be obtained.